1. Field of the Invention
The present invention concerns an automatic apparatus for carrying out the operation of welding two plate edges together by moving a welding torch along the joint separating the plate parts to be welded, or conversely by moving the plates with respect to a welding torch.
To carry out the welding operation properly, it is necessary for the axis of the welding torch to follow accurately the joint to be welded, but this joint is not necessarily perfectly straight and therefore monitoring of the position of the torch with respect to the joint has to be carried out continuously so that this position can be altered whenever necessary.
A practical example of the necessity for monitoring the position of the welding torch with respect to the joint to be welded is the case of welding edge-to-edge two cylinder elements required to be joined by welding into one single taller cylinder: tank elements are welded side by side in this way, and the welding operation is carried out with a torch which remains at a substantially fixed station while the cylindrical plates turn in front of it as welding proceeds; if the joint between the plates does not remain completely in a plane which contains the axis of the welding torch, it is necessary to move this slightly when the joint moves out of this plane.
In other instances, instead of the plates moving under the welding torch, the torch moves in the direction of the length of the joint to be welded; exactly the same problem arises and if the joint moves out of a line or a plane containing the axis of the welding torch, the torch again has to be moved laterally with respect to the joint.
2. Prior Art
The prior technique currently used to achieve this following of the joint during welding consists in providing for the torch to be mounted on a transverse slide along which it is able to move. In present day boiler-making, the torch is moved manually but this becomes a tricky operation if the precision or speed of welding has to be increased.
In plasma welding processes, guiding precision has to be very high and manual guiding by an operator is excluded in practice. Automatic guiding has consequently already been proposed, by means of a mechanical feeler which stays in permanent contact with the joint, the joint then following the lateral movements of the mechanical feeler. This is possible in cases where the edges of plates in contact are bevelled and where the joint to be welded therefore forms a sort of groove which can be easily followed by the mechanical feeler (in the form of a conical or spherical point, for example). It is also usable when the two plates to be welded overlap, the feeler being able to rest against the edge of the upper plate. It can also be used when the plates are perpendicular. However, even in these instances, use of a mechanical feeler is only satisfactory if the edges of the plates are completely regular and do not have too many faults.
In any event, in the case of unbevelled plates, and particularly whenever the plates are thin and welded edge to edge, guiding by mechanical feeler is certainly not possible.
Automatic guiding methods can therefore by envisaged which used devices for sensing the position of the joint working without contact with the joint, for example, optical sensing devices. The applicant found, by trying the replacement of mechanical feelers with such optical sensing devices, and by maintaining the principle according to which the welding torch follows the displacements of the sensing device which itself follows the joint to be welded, that very precise and, most importantly, reliable guiding was not achieved, in view of the risks of drift within the servo-control and also the risks of disengaging the servo-controls in the case of irregularity in the joint to be welded, or even merely because of the difficulty in servo-optically tracking a line which has poor or irregular contrast.